This invention relates to a process for the preparation of polytetramethylene ether glycol (PTMEG) diesters of the formula R--CO--O(--CH.sub.2 -CH.sub.2 -CH.sub.2 -CH.sub.2 -O).sub.n --COR.sub.1, wherein R and R.sub.1 are identical or different and are an alkyl radical or a derivative thereof, and n is preferably an integer from 2 to 200, by polymerization of tetrahydrofuran (THF) in the presence of carboxylic acid anhydride using a neutral or slightly basic magnesium-aluminium hydrosilicate catalyst that has been activated by heating to 200-600.degree. C. The natural minerals sepiolite and, in particular, attapulgite, are suitable raw materials for the new catalysts of this invention.
The polymerization of THF by means of oxonium ion catalysis became known as a result of the basic research work carried out by H. Meerwein et. al. (Angew. Chemie 72, (1960), 927), and is treated comprehensively in the monograph "Polytetrahydrofuran" by P. Dreyfus, Gorden and Breach Sc. Publishers, New York, London, Paris 1982.
The German patent no. 2916653 describes a polymerization process for THF--that has been purified in a separate step--in which the polymerization takes place by means of bleaching earth in a fixed bed, in the presence of carboxylic acid anhydride. Bleaching earths are naturally occurring aluminium silicates with a cryptocrystalline three-layer structure of the montmorillonite mineral. The mineral, obtained from deposits thereof, exhibits physical and chemical properties which vary depending on the origin. This applies in particular to the activity of the catalysts, which is not constant but varies from batch to batch. Despite the low price, this poses a serious disadvantage with regard to the commercial use of bleaching earths as catalysts. Kaolin and zeolites, proposed in the PCT application no. WO 94/05719 for THF poymerizatation, have similar disadvantages. These minerals do not become active catalysts until they have undergone acid treatment. An added disadvantage of these minerals is that a commercially usable polymer is only obtained if the THF is extremely pure.
The object of the present invention was thus to simplify the industrial-scale polymerization of THF and to render it reproducible, at the same time retaining the advantages--particularly that of fixed-bed catalysis--of the method described in German patent no. 2916653.
Surprisingly, it was found that granular or pelletized sepiolite and attapulgite which are largely free from water and are either neutral-to-weakly-basic (pH=7-9.5) or else protonated as a result of acid treatment, when suspended, or, better, when introduced into a stationary, fixed catalyst bed, convert a mixture of THF and carboxylic acid anhydride reproducibly into the carboxylic ester of polytetrabutylene ether glycol at a high polymerization rate, the ester so formed having a low color index and the catalyst being active over an unusually long period of time. The THF does not need to be specially purified as in the case of bleaching earths, zeolites or kaolin, and even THF containing water (e.g. 1% water) can be polymerized. Through use of attapulgite, in particular, as catalyst, polymers are obtained which have a very low color index, a narrow molecular weight distribution and an extremely low content of crown ether imputities. Attapulgite is used with preference in the process according to the invention.
The catalyst remains active for a practically unlimited period of time, this being an added reason for the better environmental compatibility of the new process.